Suicide is the 3rd leading cause of death in the US ages between 15-34 years and about one million people commit suicide every year world-wide. Thus, there is a desperate need for identifying risk factors and for non- invasive, reliable biomarkers that can be used for early detection of suicidality and treatment response. Recently, microRNAs (miRNAs) have emerged as an important class of small non-coding RNAs that bind to 3' UTR of mRNAs and suppress the translation and/or stability of specific target genes. Since miRNAs show a highly regulated expression, they contribute in the development and maintenance of a specific transcriptome and thus have the unique ability to influence physiological and disease phenotypes. Our recent studies show that the expression of a group of miRNAs is altered in brain of depressed subjects and that they are involved in coping response to stress. In addition, our preliminary data indicate that a subset of miRNAs is specifically altered in brain of suicide subjects regardless of psychopathology, suggesting that miRNAs can distinguish suicidality. Recently, circulating miRNAs are under intense investigation and have been extremely useful in detecting and following the course of various diseases. Neural miRNAs are responsive to environmental, synaptic, and pathological changes and can be actively secreted by cells such as exosomes from brain into blood. These exosomes bear cell-type specific surface markers. Using a neural specific surface marker, we successfully isolated neural-derived exosomes and found that these exosomes are enriched with miRNAs/mRNAs that are expressed in brain. Using this novel approach we aim to examine whether neural- derived exosomal miRNAs are differentially expressed that are specific to suicidal ideation or behavior, and which by affecting specific mRNA targets and pathways, are associated with suicidal behavior and response to ketamine. We will examine the following groups of subjects: 1) major depressive disorder (MDD) with a recent suicide attempt (in past 2 weeks), 2) MDD with serious ideation without recent suicide attempt (in the past 6 months), 3) MDD without clinically significant suicidal ideation or suicide attempt in the past 6 months, and 4) healthy controls. Both suicidal and non-suicidal MDD will be given ketamine (0.5 mg/kg, IV) and blood drawn at 30 and 180 min post-infusion to measure changes in miRNAs. We also propose a parallel human postmortem brain study to examine whether changes in miRNAs in suicidality correspond to miRNA changes in brain by comparing dlPFC and hippocampus from MDD suicide, MDD non-suicide, and control subjects. With this we attempt to achieve: 1) whether suicidal ideation or behavior is associated with differences in the expression of specific miRNAs, 2) whether anti-suicidal/antidepressant effects of ketamine is associated with miRNAs changes, and 3) whether miRNA/mRNA-regulatory pathways contribute to suicide pathogenesis and treatment response. Our study will provide a novel avenue for the development of miRNAs as ''molecular tool'' to identify suicidality and treatment response and in generating target based therapies to treat this devastating disorder.